The system has been used since January 2014 to treat patients. Based on the results of ongoing machine and patient-specific tests and with independent confirmation, the treatments are being delivered accurately and safely.
Olga L. Green, PhD, H.O. Wooten, PhD, Yanle Hu, PhD, Rojano Kashani, PhD, Lakshmi Santanam, PhD, Harold Li, PhD, Sasa Mutic, PhD; Washington University School of Medicine
PURPOSE: We describe the potential problems and solutions developed in initiating first treatments with a commercially available magnetic resonance image–guided radiation therapy (MR-IGRT) system. Specifically, we address the following: (1) effect of magnetic fields on dose distributions, (2) patient and staff safety in the presence of magnetic and radiation fields, and (3) quality assurance for MR-IGRT.
MATERIALS AND METHODS: The foremost problem of integrating an MRI system with a radiation delivery system has been solved by combining a 0.35-T split-doughnut MRI with a gantry carrying three 60 Co sources. This system was installed in our institution in 2012, and since then, we have conducted ongoing work toward its clinical implementation. The problem of the effect of magnetic fields on dosimetric distributions was investigated by comparing measurements with an in-house phantom to Monte Carlo calculations. The problem of patient and staff safety was addressed by measuring the specific absorption ratio (SAR) to ensure that it was safe for patients to be imaged continuously while being treated and by implementing safety checks in our daily workflow. The problem of quality assurance for MR-IGRT was solved by establishing the accuracy of measurement devices (ionization chambers, detector arrays) in the presence of the magnetic field and developing tests that checked both the geometric and dosimetric accuracy of the system on a daily, weekly, monthly, and annual basis. Independent confirmation of dosimetric accuracy was provided by the Imaging and Radiation Oncology Core (IROC) service-reference dosimetry via optically stimulated dosimeters and overall delivery quality via the head-and-neck phantom (film and thermoluminescent dosimeters).
RESULTS: The 0.35-T magnetic field was found to have a negligible effect on dose distributions for the type of patient plans used clinically. The measured SAR value was 1.14 W/kg, which ensured that patients do not experience excessive heating during the ongoing real-time imaging at four frames per second. MR training was conducted for all staff, and procedures were implemented to ensure that patients were cleared to be MR-safe before every treatment. Quality assurance included spatial integrity and homogeneity of the MR field, MR and RT isocenter coincidence, and pre- and posttreatment patient-specific QA. Satisfactory results were reported by the IROC for four independent OSL (Optically Stimulated Luminescence) reference dosimetry checks and the head-and-neck phantom.
CONCLUSION: The system has been used since January 2014 to treat patients. Based on the results of ongoing machine and patient-specific tests and with independent confirmation, the treatments are being delivered accurately and safely.
Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org